Optical elements made from ThF4 --BeF2 glasses

ABSTRACT

ThF4-BeF2 glasses of specified composition exhibit sufficiently low high-temperature viscosity and melt stability to be useful for the fabrication of optical devices for ultraviolet or infrared light transmission. The low melt viscosity renders the glasses suitable as host materials for rare earth and transition metal dopants so that they can be used for optical devices such as lasers and optical filters.

The Government has rights in this invention pursuant to Contract No.N00014-82-C-2314 awarded by the Department of the Navy.

This application is a continuation-in-part of copending application Ser.No. 06/674,411, filed Nov. 23, 1984 and now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to glass compositions based on BeF₂ andparticularly to ThF₄ --BeF₂ glasses exhibiting improved meltingproperties and an extended melting range, rendering them particularlysuitable for infrared or u.v.-transmitting optical devices.

In recent years there has been considerable interest in fluoroberyllateglasses, primarily for applications in laser technology. Very lowrefractive index, dispersion power, and non-linear coefficients ofrefractive index are among the properties that are unique to this familyof glasses. In addition, fluoroberyllate glasses possess betterultraviolet transmission than fused silica and are good hosts forfluorescing elements such as the rare earths. The primary drawbacks tothese glasses are the toxicity of beryllium and the poor waterdurability of BeF₂ -based compositions.

Besides laser applications, the low dispersion characteristics and longrange infrared transparency of these glasses make them good candidatesfor ultra-low-loss optical waveguides. In a cladded optical waveguidestructure the importance of core durability is minimal if the claddingcan be formed of a durable glass.

Many possible variations of BeF₂ compositions appear in the literature,but nearly all of these require relatively large amounts of a second orthird component to achieve the necessary softening of the glass. Rawson,"Inorganic Glass-Forming Systems", Academic, New York (1967) pages236-247, provides a good background description of the known BeF₂glass-forming systems, while U.S. Pat. No. 3,650,779 describesalkali-fluoroberyllate glass used as a host matrix for xenon.

In U.S. Pat. No. 3,958,970, MgF₂ --PbF₂ --BeF₂ glasses are used to makefluorescing materials via additions of ytterbium fluoride, erbiumfluoride and thulium fluoride. U.S. Pat. No. 4,142,986 disclosesNaF--AlF₃ --BeF₂ glasses containing NaPO₃ and doped with NdF₃ to providean infrared laser glass, while similarly doped glasses based in the BeF₂--AlF₃ composition system are described in published French applicationNo. 2,445,820.

The list of additives to BeF₂ -based glasses has included, in additionto the modifying alkali and alkaline earth metal fluorides, lanthanideseries elements, cerium, and thorium. U.S. Pat. Nos. 2,466,507 and2,511,224 disclose complex optical glasses of this type.

As noted by Rawson, supra, BeF₂ forms a glass by itself and has astructure which is analogous to fused silica. However, the meltingcharacteristics of pure BeF₂ are not well suited to the preparation ofhomogeneous glasses containing minor additions of modifyingconstituents, because of the high viscosity and short working range ofBeF₂ glass. In order to effectively mix BeF₂ melts, particularlycrucible melts, viscosities below 100 poises are needed. To achievethese viscosities in pure BeF₂ melts ordinarily requires heating totemperatures above 970° C., and at these temperatures the vapor pressureof BeF₂ becomes appreciable. It is this characteristic which hasnecessitated the development of the complex BeF₂ -containing glasscompositions known in the prior art.

Phase equilibrium studies of the systems BeF₂ --ThF₄ and LiF--BeF₂--ThF₄ are reported by R. E. Thoma et al. in J. Phys. Chem., 64 (July1960) 865-870. However, Thoma et al. characterized the BeF₂ melts ashigh in viscosity. No information is provided as to the clarity of themelts prepared for the study, nor as to any of the other materialproperties of the system.

In any glass composition field, it is advantageous to be able to obtaindesired property modifications with as small a composition excursion aspossible. In BeF₂ glasses, small additions of alkali can be toleratedwithout destroying clarity; however, other additions such as PbF₂ have astrong destabilizing effect and do not produce useful glasses. Bothsystems have been described by D. M. Ray et al., J. Am. Cer. Soc., 37(7)300-305 (1954).

This consideration is particularly critical for optical applicationssuch as optical fibers wherein substantial modification of opticalconstants such as refractive index must be achieved with very smallquantities of additives, in order to avoid large shifts in physicalproperties and, most importantly, phase separation of any kind. Opticalfibers provide extremely large optical path lengths wherein even theslightest crystallization or phase separation destroys utility.

A principal object of the present invention is to simplify theproduction of optical devices comprising BeF₂ -based glasses through theuse of glass compositions of improved stability and reduced viscosity.

It is a further object of the invention to adapt BeF₂ -based glasscompositions comprising only simple additions of a singleviscosity-modifying constituent to the fabrication of BeF₂ glass opticaldevices.

Other objects and advantages of the invention will become apparent fromthe following description.

SUMMARY OF THE INVENTION

The present invention comprises the manufacture of glass optical devicesfrom ThF₄ --BeF₂ glass compositions. These compositions have been foundto demonstrate excellent glass melting behavior, and can therefore canbe used to produce clear ThF₄ --BeF₂ glass articles of good quality bycasting, drawing or other conventional glass-forming techniques.Compositions according to the invention consist essentially, in molepercent, of about 3-30% ThF₄ and 70-97% BeF₂. Within this compositionrange clear, water-white glasses are typically formed, whilecompositions containing more than 30 mole percent ThF₄ exhibit slight tomoderate phase separation and/or devitrification, and can exhibitvarying degrees of haze or translucence depending upon composition andthe forming process which is used.

In addition to providing high quality base glasses, the ThF₄ --BeF₂compositions are generally compatible with additives such as thetransition metal and rare earth elements. Thus while the pure ThF₄--BeF₂ glasses may be used for optical fiber elements or elemets foru.v. or infrared light transmission, glasses doped with additionalelements are useful for applications wherein active or passive opticalcomponents such as optical filters, glass luminescent or fluorescentelements, or laser glasses elements are needed. Examples of elementswhich can be included in these glasses include any of the rare earth orLanthanide Series elements of the period Table (atomic numbers 58-71),including for example Ce, Sm, Er, Tb, Pr, Gd and Dy. Transition metaladditives may include elements of atomic numbers 21-30, such as iron andcopper.

DESCRIPTION OF THE DRAWINGS

The invention may be further understood by reference to the drawings,wherein:

FIG. 1 is a plot of glass viscosity as a function of temperature for aBeF₂ glass; also shown is an estimated viscosity for a representativeThF₄ --BeF₂ glass melt;

FIG. 2 includes specific absorbance plots of a BeF₂ glass and of a ThF₄--BeF₂ glass of the invention;

FIG. 3 is a specific absorbance plot of a ThF₄ --BeF₂ glass doped witherbium; and

FIG. 4 is a specific absorbance plot of a ThF₄ --BeF₂ glass doped withiron.

DETAILED DESCRIPTION

Because of the toxicity of beryllium and the fact that BeF₂ glasses tendto be hygroscopic, an enclosed melting facility for preparing ThF₄--BeF₂ glasses is desirable. A suitable apparatus for preparing glassesaccording to the invention is an atmospherically controlled glove boxmelter wherein crucible melts may be prepared and the resulting glasseshandled and stored in a dry, inert environment.

Batches for the glasses may be compounded from fluorides or othercompounds of the cationic constituents in proportions which will yieldthe desired glass compositions at the temperature utilized for meltingthe batch. The preferred beryllium-containing batch material is ammoniumberyllium fluoride, (NH₄)₂ BeF₄, while ThF₄ may be used as the thoriumbatch ingredient. Rare earth and transition metal additives, ifincluded, may also be introduced as fluorides. For additives introducedin small proportions chlorides or other halides may be used; thepresence of minor amounts of chloride does not adversely affect glassquality.

Glass batches compounded according to the invention may be readilymelted at temperatures of about 900°-1000° C. Melt viscosities at 950°C. are suitable for homogenizing the melt by swirling or stirring, andglass articles may be formed from the melt by drawing, rolling, castingor any other conventional method. The melts are typically of goodquality, and clear, water-white glass articles can easily be formed bycasting.

Table I below reports glass compositions useful for making opticaldevices within the scope of the invention, together with a comparativeBeF₂ glass. Included in Table I are compositions for each glass, in molepercent as calculated from the batch, and batch compositions in parts byweight. Also included in Table I are indications of the characteristicsof the melt, whether fluid or viscous, and the characteristics of glasspatties formed by slow cooling from the melt at thickness of 5-8 mm.

Batches for the glasses reported in Table I were compounded using ThF₄of 99.9% purity and (NH₄)₂ BeF₂ of 99% purity, the latter beingcommercially obtainable from Atomergic Chemetals Corp. of Plainview,N.Y. The batches were hand-mixed, and the mixed batches were then meltedin vitreous carbon crucibles under a dry nitrogen atmosphere in aplatinum-wound electric furnace operating at 950° C. The melts werehand-stirred once during melting.

After melting, the crucibles were removed from the furnace and the glasswas allowed to cool in the crucibles to room temperature, after whichthe glass samples were easily removed. The glasses were ground andpolished under kerosene, and were stored under kerosene to avoidmoisture pick-up when not in use.

                                      TABLE I                                     __________________________________________________________________________    Glass and Batch Compositions                                                         1    2   3    4   5    6   7                                           __________________________________________________________________________    Glass                                                                         Composition                                                                   BeF.sub.2                                                                            95   97  90   80  70   60  100                                         ThF.sub.4                                                                            5    3   10   20  30   40  --                                          Batch                                                                         Composition                                                                   (NH.sub.4).sub.2 BeF.sub.4                                                           23.0 23.5                                                                              21.8 19.3                                                                              4.2  3.60                                                                              25.00                                       ThF.sub.4                                                                            3.1  1.9 6.2  12.3                                                                              4.6  6.10                                                                              --                                          Properties                                                                    Melt   Fluid                                                                              Fluid                                                                             Fluid                                                                              Fluid                                                                             Fluid                                                                              Fluid                                                                             Viscous                                     Glass  Clear                                                                              Clear                                                                             Clear                                                                              Slight                                                                            Hazy Slight                                                                            Clear                                       Quality              Haze     Devit.                                          __________________________________________________________________________

While the pure BeF₂ glass produced in accordance with the aboveprocedure (Example 7) was quite viscous, as little as 3 mole percentadded ThF₄ (Example 2) reduced the viscosity of the melt such thateffective mixing and casting were easily accomplished. FIG. 1 of thedrawing is a viscosity temperature curve for a pure BeF₂ glass such asExample 7 showing a viscosity at 950° C. of about 150 poises. Incontrast, the estimated 950° C. viscosity for the 3% ThF₄ -97% BeF₂glass of Table I, also shown in FIG. 1, is less than 50 poises.

No phase separation or ThF₄ loss from these melts was observed at ThF₄concentrations up to 10 mole percent. Some ThF₄ volatilization occuredat concentrations above 10 mole percent, some phase separation wasobserved at ThF₄ concentrations above 12 mole percent. However, goodquality clear-to-slightly opal glasses are obtainable at ThF₄concentrations (batched) about 30 mole percent, and up to 40 molepercent ThF₄ if rapid-quench forming techniques such as rolling, ratherthan slow-cooling forming methods, are used.

In addition to improving the melting characteristics of the glass, ThF₄additions appear to improve somewhat the water durability of the glass,to shift the ultraviolet absorption edge of the glass to somewhat longerwavelengths, and to shift the infrared absorption edge to somewhatlonger wavelengths. FIG. 2 of the drawing sets forth composite specificabsorbance plots for a BeF₂ glass and for a ThF₄ --BeF₂ glass containing12 mole percent ThF₄ wherein these spectral effects are shown.

Referring to FIG. 2, the infrared spectrum from 2.5-5 microns (measuredwith a Perkin-Elmer Model 681 infrared spectrophotometer) shows a strongabsorption peak at 2.7 microns attributed to the presence of OH groupsin the glass. The presence of OH is also known to effect the position ofthe ultraviolet absorption edge of BeF₂ -based glasses. Hence in thespectral region between 0.25 and 2.5 microns (measured with a Cary Model17 spectrophotometer) wherein another OH peak is observed at 1.4microns, the ultraviolet absorption edges of both glasses are positionedat longer wavelengths than would be found in OH-free glasses of the samefluoride composition. The decrease in absorbance for the ThF₄ --BeF₂glass in the visible to near-infrared spectral region is attributed toimproved glass surface quality, rather than to any real improvement inglass transmission.

Due to the reduced viscosity of the melts, the homogeneous inclusion ofminor amounts of rare earth or transition metal dopants in these ThF₄--BeF₂ glasses can be easily accomplished, and these additions impartclear characteristic spectral absorption features to the doped glasses.FIG. 3 of the drawing sets forth a specific absorbance plot for a ThF₄--BeF₂ glass containing 3200 ppm by weight of erbium as a dopant. Thebase glass had the composition of Example 2 in Table I above, including3 mole percent ThF₄ ; the Er dopant was introduced to the glass batch aspure ErF₃. The absorption spectrum of this glass, including well-definedabsorption peaks characteristic of the particular rare earth dopantselected, is representative of the types of absorption behaviorexhibited by rare earth-doped ThF₄ --BeF₂ glasses of the invention.

The same base glass (97% BeF₂ --3% ThF₄) was doped with a number ofother rare earth elements to evaluate the characteristics of theseglasses as suitable host materials. Table II below reports opticalabsorption data for six rare earth dopants at selected wavelengths whererelatively strong characteristic absorption peaks appeared in theabsorption spectra of the doped glasses. Included in Table II for eachdopant are the doping concentration level, in weight percent, thewavelengths (nm) of the absorption peaks measured, the energy transitiongiving rise to each peak, and the absorption coefficient at the peakwavelength due to the presence of the dopant in the specifiedconcentration.

                  TABLE II                                                        ______________________________________                                        Optical Absorption in                                                         Doped ThF.sub.4 - BeF.sub.2 Glass                                                       Doping                     Absorption                                         Level    Absorption                                                                              Electron                                                                              Coeff.                                   Dopant    (% Wt.)  Peak (nm) Transition                                                                            (cm.sup.-1)                              ______________________________________                                        Dysprosium                                                                              0.39     348       .sup.4 G.sub.11/12                                                                    0.1105                                                      1070      .sup.9/2 H.sub.7/2                                                                    0.0829                                                      1265      .sup.11/12 H.sub.9/2                                                                  0.2648                                   Gadolinium                                                                              0.18     272       .sup.6 I.sub.13/2                                                                     0.023                                                       275       .sup.6 I.sub.11/12                                                                    0.033                                    Samarium  0.27     397       4K.sub.11/12                                                                          0.1221                                                      1070      .sup.9/2 H.sub.15/2                                                                   0.1382                                                      1270      .sup.7/2 H.sub.15/2                                                                   0.1681                                   Erbium    0.32     376       .sup.4 G.sub.11/12                                                                    0.4353                                                      517       .sup.3 H.sub.11/12                                                                    0.2764                                                      647       .sup.4 F.sub.9/2                                                                      0.0668                                   Terbium    0.435   269       .sup.3 D.sub.2                                                                        0.0921                                                      1910      .sup.7 F.sub.11/12                                                                    0.0852                                   Praseodymium                                                                             0.214   438       .sup.3 P.sub.2                                                                        0.1819                                                      1515      .sup.3 F.sub.4                                                                        0.1382                                   ______________________________________                                    

For all of the above dopants, absorption data similar to that reportedabove was used to establish that Beer's Law is obeyed in these glasses,ie., a linear dependance between absorption and the concentration of theabsorber is observed. A cerium dopant was also tried but, unlike theabove dopants, it did not produce strong absorption peaks in theseglasses. However, both the infrared and the ultraviolet absorption peakswere shifted to longer wavelengths in the cerium-doped glass.

Transition metal doping of these glasses suggests that BeF₂ creates astrongly reducing environment. Additions of copper (II) chloride (CuCl₂)produces broad absorption at 700 nm, attributed to divalent Cu; chemicalanalysis indicates the Cl is preferentially volatilized from the melt.However, the intensity of the 700 nm absorption peak is nearlyindependent of copper concentration over the range from 0.1-0.36% byweight, suggesting that most of the copper is reduced to the monovalentstate.

Similarly, iron dopants in these glasses produce an absorptioncharacteristic of Fe⁺² regardless of whether ferrous iron or ferric ironis introduced. FIG. 4 of the drawing is a specific absorbance plot for aglass having the composition of Example 2 of Table I, containing 97 molepercent BeF₂ and 3 mole percent ThF₄, to which was added 1130 ppm(weight) of iron as FeF₃. The broad absorption peak centered at 1080 nmis attributed to Fe⁺² absorption, and the absorption of iron followsBeer's law.

From the foregoing it is concluded that ThF₄ --BeF₂ glasses provide asuitable host material not only for rare earth dopants, but also fortransition metal dopants. In either case, utilizing a preferred range ofbase glass compositions consisting essentially, in mole percent, of3-12% ThF₄ and 88-97% BeF₂, the homogeneous addition of up to 1% byweight total of rare earth and/or transition metal dopants can easily beaccomplished for optical device applications requiring such dopants.

While other binary systems such as PbF₂ --BeF₂ and BaF₂ --BeF₂ encompasslow-melting eutectics as does the ThF₄ --BeF₂ system, such eutectics donot guarantee good glass forming behavior. To the contrary, additions ofThF₄ appear uniquely effective to modify the high temperature viscosityof BeF₂ melts without destabilizing the composition or promoting thedevelopment of phase separation or crystallization. This behavior is inmarked contrast to other common fluoride modifiers which have been usedin BeF₂ -based glasses. Table III below reports data resulting fromglass melting studies concerning the effects of various known modifierson melt viscosity and melt and glass quality in BeF₂ -based glasses.Included in Table III for each of a number of glass melts are thecomposition of each melt, in mole percent, the appearance of the melt at950° C., and the optical quality of glass patties formed by casting andcooling the melts.

                  TABLE III                                                       ______________________________________                                        Modified BeF.sub.2 Glass Melts                                                Melt Composition                                                              (mole %)      Melt Appearance                                                                            Glass Quality                                      ______________________________________                                        BeF.sub.2     very stiff, grey                                                                           grey glass,                                                                   wouldn't pour                                      98.5BeF.sub.2 --1.5AlF.sub.3                                                                stiff, grey  grey glass                                         79% BeF.sub.2 --21ZrF.sub.4                                                                 very stiff   devitrified                                        90BeF.sub.2 --10AlF.sub.3                                                                   wouldn't pour                                                                              opaque glass                                       90BeF.sub.2 --10PbF.sub.2                                                                   clear, fluid crystallized                                       92BeF.sub.2 --8PbF.sub.2                                                                    clear, stiff crystallized                                       85BeF.sub.2 --15PbF.sub.2                                                                   clear, very fluid                                                                          crystallized                                                                  very rapidly                                       90BeF.sub.2 --5PbF.sub.2 --5AlF.sub.3                                                       clear, fluid hazy glass                                         90BeF.sub.2 --7AlF.sub.3 --3PbF.sub.2                                                       clear, fluid clear glass,                                                                  some crystallization                               90BeF.sub.2 --10BaF.sub.2                                                                   clear, fluid hazy glass                                         90BeF.sub.2 --10LaF.sub.3                                                                   clear, fluid devitrified                                        90BeF.sub.2 --5LiF--5PbF.sub.2                                                              clear, fluid devitrified                                        90BeF.sub.2 --10HfF.sub.4                                                                   clear, fluid hazy glass                                         90BeF.sub.2 --10GaF.sub.3                                                                   didn't melt  --                                                 90BeF.sub.2 --10ZnF.sub.2                                                                   clear, fluid opal glass                                         97BeF.sub.2 --3ThF.sub.4                                                                    clear, fluid clear glass                                        95BeF.sub.2 --5ThF.sub.4                                                                    clear, very fluid                                                                          clear glass                                        ______________________________________                                    

As indicated by the data in Table III above, while some heavy metalfluorides or combinations thereof such as PbF₂, BaF₂ and the like areeffective to reduce the high temperature viscosity of BeF₂ -based melts,(e.g., the viscosity at 950° C. which is of particular interest formelting these glasses), each of these commonly used fluorides exhibits adestabilizing effect on the glass, and thus hinders the production ofproducts of optical quality. It is only ThF₂ which appears capable ofreducing melt viscosity without adversely affecting the stability ofBeF₂ as a glass former. Further, these reductions in melt viscosity areobtained without major changes in other glass properties, such asrefractive index, which changes are necessary consequences of using therelatively large quantities of conventional modifiers which are requiredto obtain stable BeF₂ melts of reduced viscosity. This makes the use ofThF₂ --BeF₂ glasses in accordance with the invention to fabricateoptical devices, and particularly devices incorporating homogeneouslydispersed additions of rare earth elements such as herein describedespecially valuable.

The fabrication of optical devices from ThF₄ --BeF₂ glasses according tothe invention may be accomplished by any known method. Optical elementssuch as prisms or lenses may be formed by molding, with subsequentpolishing to provide surfaces of optical quality. Plates for opticalfilters may be provided by casting, drawing, or rolling. Rod preformsfor laser rods or optical fiber preforms may be formed by casting ordrawing, with optical fiber formed by redrawing from the preform rod.Alternatively, optical fiber may be formed by direct drawing fromcrucible melts. Of course, these procedures are merely illustrative ofthe various known methods by which optical devices could be formed fromThF₄ --BeF₂ glasses according to the invention.

I claim:
 1. A glass laser rod formed of a transparent, crystal-freeglass having a composition consisting essentially of 3-30% mole percentThF₄, 70-97 mole percent BeF₂ and a rare earth dopant in an amountranging up to about 1% by weight of the glass.
 2. The glass laser rod inaccordance with claim 1 wherein the ThF₄ content is 3-12 mole percentand the BeF₂ content is 88-97 mole percent.
 3. A glass optical fibercomprising a transparent, crystal-free glass core having a compositionconsisting essentially of 3-12 mole percent ThF₄ and 88-97 mole percentBeF₂.
 4. A method of making a glass optical fiber consistingpredominantly of BeF₂ which comprises the step of:(a) adding a ThF₄modifier to a batch for a BeF₂ glass in an amount effective to reducethe 950° C. viscosity of the resulting BeF₂ --ThF₄ batch composition toa value below that of pure BeF₂ ; (b) heating the resulting BeF₂ --ThF₄batch composition to form a homogeneous molten glass; (c) drawing aglass rod preform for the glass optical fiber from the molten glass; and(d) redrawing the preform rod into a glass optical fiber.
 5. A method inaccordance with claim 4 wherein the molten glass consists essentially of3-12 mole percent ThF₄ and 88-97% BeF₂.